Image copyrightPhil Briggs/BCTImage caption
Anyone can take part in the monitoring programme

These volunteers have been out across the nation counting the animals using detectors that pick up their ultrasonic chatter.

The echolocation sounds not only announce the bats' presence but can also be used to distinguish the different species.

Those doing particularly well include the greater and lesser horseshoe bats; those apparently not doing quite so well include the brown long-eared bat and the serotine bat.

Image copyrightHugh ClarkImage caption
Nathusius' pipistrelle: Bats are seen as an indicator of the general health of ecosystems

But the figures carried some caveats, said Mr Briggs.

Natural year-on-year variability has to be accounted for, and for some species - such as the Leisler's bat and the Nathusius' pipistrelle - there is simply insufficient data on which to make a definitive statement.

It is the case, too, that some animals are harder to count because of their behaviour, and this may underplay their true status.

"Some of the woodland species, for example, have quiet calls so they're more difficult to pick up on bat detectors, and they can be hard to tell apart because their calls are quite similar," Mr Briggs explained.

"Sometimes the only way you can tell the difference is when experienced volunteers trap the bats and examine them."

The Bechstein’s and barbastelle bats fall into this category. One innovation for the future that may help is automated listening stations that will record sounds that can then be analysed in detail by computer.

Bat numbers in the 20th Century were greatly reduced because of human activity.

Deleterious impacts included roost and habitat loss, disruption of insect food supplies, increased urbanisation, and the general spread of artificial lighting.

Image copyrightGareth JonesImage caption
The greater horseshoe bats appear to be adding to their population successfully

More recent developments such as wind turbines have also put pressure on populations.

The power units' installation had to be planned and managed with care, said Mr Briggs.

"It's thought bats may be drawn to turbines, to use them as features on the landscape to help them navigate.

"They don't necessarily get killed by collision. When they get close to the blades, just the change in air pressure can cause internal injuries. Wind turbines are obviously very important but they need to be sited away from important bat commuter routes."

Anyone wishing to get involved in National Bat Monitoring Programme Surveys can enrol online.